Diversity transmitter and diversity transmission method

ABSTRACT

The present invention concerns a diversity transmitter, comprising: transmit symbol input means ( 1 ) for inputting a symbol matrix (b) to be forwarded to a transmit processing means ( 2 ), said transmit processing means comprising supplying means ( 2   a ) for supplying columns of said symbol to a plurality of at least two branches, each branch being supplied to a respective one of spatial channels (A 1 , . . . , Am) for transmission to a receiver, a parallelization means ( 2   b ) adapted to provide within each branch at least two parallel channels allocated to a respective user, and weighting means ( 2   c ) adapted to subject the symbol signals on at least one of said branches to an invertible linear transformation with a fixed complex weight, the complex weight being different for at least two parallel channels. The present invention also concerns a corresponding diversity transmission method.

FIELD OF THE INVENTION

The present invention relates to diversity transmitters, and inparticular to diversity transmitters for use in connection with mobilecommunication systems such as UMTS and the like. Also, the presentinvention relates to a corresponding diversity transmission method.

BACKGROUND OF THE INVENTION

In connection with diversity transmitters, different concepts are beingdiscussed. In general, so called open-loop concepts and closed-loopconcepts can be distinguished, as it is for example outlined in thedocument “A Randomization Technique for Non-Orthogonal Space-Time BlockCodes” by the present inventor and his co-author, presented on IEEEVehicular Technology Conference, May 2001, Rhodes, Greece.

A number of different such open-loop concepts have been proposed in3^(rd) generation partnership project 3GPP (and/or 3GPP2). For example,in the above mentioned document “A Randomization Technique forNon-Orthogonal Space-Time Block Codes” Applicants have presented the socalled ABBA concept in 3GPP2. Motorola has proposed a combination ofSTTD+OTD (Space-Time Transmit Diversity+orthogonal Transmit Diversity),and recently in the TSG-RAN Working Group 1 meeting #15 in Berlin,Germany, Aug. 22-Aug. 25, 2000, Samsung proposed a 2xSTTD concept in thesubmitted document “New CPICH Transmission Scheme for 4-antenna transmitdiversity”.

In the document “A Space-Time Coding Concept for a Multi-ElementTransmitter”, by the present inventor and his co-authors, presented inthe Canadian Workshop on Information Theory, June 2001, Vancouver,Canada, Applicants proposed a so-called Trombi-concept. (US-patentapplication filed on Mar. 28, 2001). Nokia's Trombi concept (to beexplained later in some greater detail) is considered by the inventor toshow currently the best performance. However, up to now theTrombi-concept was mainly implemented in connection with phase-hoppingor phase sweeping arrangements. Phase-hopping and phase sweeping can beused also in the context of the present invention, but with theTrombi-method the transmission methods involving very high data rates inWCDMA downlink can be further enhanced.

Further transmit diversity concepts have been considered in the OFDMliterature (orthogonal frequency division multiplexing). For example,such concepts are discussed in the document “Spatial Transmit diversitytechniques for broadband OFDM systems” by S. Kaiser, published in IEEE,2000, page 1824-1828, (0-7803-6451-1/00).

This proposed concept by Kaiser however requires interleaving overmultiple frequencies for full benefit. (A similar approach beingdiscussed in U.S. Pat. No. 6,157,612). Moreover, according to theteaching of Kaiser, a symbol to be transmitted is distributed acrossseveral carriers, so that for combining the received multipathcomponents, guard intervals are required in order to be able tocorrectly combine the transmitted (distributed) symbol parts at thereceiving side

Referring back to the above mentioned so-called Trombi concept thefollowing was proposed. A time-varying/hopping phase (e.g.pseudo-random) is added to the dedicated channel of a given user at theoutput of STTD encoder (Space-Time Transmit Diversity) (or an encoderbased on some other orthogonal or non-orthogonal concept, see e.g. theprevious “Randomization technique . . . ” paper).

In one solution with 4 antennas, antennas 2 and 4 are multiplied by acomplex coefficient (constant for two space-time coded [successive]symbols) to result in the following received signal (note that thereceived signal r, the symbols S, the transmission channel transferfunctions h and complex coefficients w are generally given in matrixnotation)r _(r1) =S ₁(h ₁ +w ₁(t)h ₂)−S ₂*(h ₃ +w ₂(t)h ₄)r _(r2) =S ₂(h ₁ +w ₁(t)h ₂)+S ₁*(h ₃ +w ₂(t)h ₄)  (1)

In a preferred arrangement, it is configured such that w1(t)=−w2(t),with constant amplitude=1. Phase changes according to a suitablepseudo-random sequence. For example, it can hop with phases 0, 180, 90,−90, (or with any other sequence) known [a priori] to the terminal(receiver). 8-PSK hopping appears to be sufficient to get achievablegains.

Then, the terminal estimates the channels h1, . . . , h4, for exampleusing common channel pilots (or dedicated pilots) which do not need toapply phase dynamics (e.g. common channel measurements can be done asproposed by Samsung in the cited document). Alternatively, the terminalcan measure the effective channels h1+w*h2 and h3−w*h4 only.

By knowing the channels and the pseudo-random weights at the transmitterthe intentional phase dynamics can be taken into account and then thedetection reduces to conventional STTD decoding without any complexityincrease.

In essence, the dynamics of the phase-hopping should be a priori fixedor at least it should be known by the UE (e.g. by suitable signalingfrom the transmitter to the receiver). In some cases it may also beadvantageous if the UE controls the phase-hopping sequence. As such acontrol procedure is expected to be known to those skilled in the art,these details are supposed to be not needed to be explained here.

With channel coding, providing time diversity, the concept has betterperformance in low Doppler channels than a two antenna STTD concept, asshown in the “Trombi paper”. Phase-hopping diversity can be used also ina way such that the channel estimates are directly taken from aphase-hopping channel. In that case the hopping sequence can have onlyincremental changes, as otherwise the effective channel is changing toorapidly to enable efficient channel estimation. However, in this casethe receiver terminal (User Equipment UE in UMTS) does not necessarilyneed to know that phase-hopping is used at all.

Therefore, in the aforementioned scheme, phase-hopping can weakenchannel estimation performance by the abrupt phase hops, or the hopshave to be quantized to many levels, to thereby approximate aphase-sweep.

The Trombi concept is designed for sequential transmission, and thephase-hopping sequence is defined over multiple time instants, coveringmultiple space-time encoded blocks. In future communication systems thewhole information frame may be transmitted in one or a few symbolintervals (e.g. if in a CDMA system essentially all downlink codes areallocated to one user at a time). In such an extreme case, only one or afew phase-hopping values can be incorporated to the transmission, andthe benefits of the Trombi concept cannot be achieved.

As an example, in “Draft Baseline Text for Physical Layer Portion of the1xEV Specification” 3GPP2 C.P9091 ver. 0.21, Aug. 24,2000 (3GPP2 TSG-Cworking group III) the physical layer of the High Data Rate CDMA systemis described. This system uses Time Division Multiplexing in downlinkand each user can be allocated only one slot, and the pilots arestructured so that only one channel estimate can be obtained for thisone slot.

SUMMARY OF THE INVENTION

Hence, it is an object of the present invention to provide an improveddiversity transmitter and diversity transmission method which is freefrom the above mentioned drawbacks.

According to the present invention, this object is for example achievedby a diversity transmitter, comprising: transmit symbol input means forinputting a symbol matrix to be forwarded to a transmit processingmeans, said transmit processing means comprising supplying means forsupplying columns of said symbol matrix to a plurality of at least twobranches, each branch being supplied to a respective one of spatialchannels for transmission to a receiver, a parallelization means adaptedto provide within each branch at least two parallel channels allocatedto a respective user, and weighting means adapted to subject the symbolmatrix signals on at least one of said branches to an invertible lineartransformation with at least one fixed complex weight, the complexweight being different for at least two parallel channels.

According to the present invention, this object is for example alsoachieved by a diversity transmission method, comprising the steps ofinputting a symbol matrix for being processed, said processingcomprising supplying columns of said symbol matrix to a plurality of atleast two branches, each branch being supplied to a respective one ofspatial channels for transmission, performing parallelization so as toprovide within each branch at least two parallel channels allocated to arespective user, and subjecting the symbol matrix signals on at leastone of said branches to an invertible linear transformation with atleast one fixed complex weight, the complex weight being different forat least two parallel channels.

According to further refinements of the present invention (method aswell as transmitter),

-   -   said invertible linear transformation is a unitary        transformation,    -   said unitary transformation is represented by a unitary weight        matrix in which at least two elements have different non-zero        complex phase values,    -   said parallelization means/step is adapted to perform multicode        transmission using multiple spreading codes,    -   multicode transmission is performed using a Hadamard        transformation by multiplying the symbols with a spreading code        matrix H,    -   said spreading code matrix is antenna specific,    -   said spreading codes are non-orthogonal spreading codes,    -   said spreading codes are orthogonal spreading codes,    -   said fixed complex weights applied by said weighting means/step        are time-invariant phase shift amounts for the respective        parallel channels,    -   the phase shift amounts are independent of the channels in at        least two corresponding parallel channels transmitted out of        different antennas,    -   the phase shift amounts are dependent on the channels,    -   said weighting matrix is identical for each branch,    -   said weighting matrix differs for each branch.    -   there is provided a pre-diversification step/means performed        after/arranged downstream inputting and performed        before/arranged upstream processing, said pre-diversification        step/means subjecting said inputted symbol sequence to a        diversification, at least one diversified symbol sequence being        subjected to said processing,    -   said pre-diversification step/means subjects said input symbol        sequence to at least one of an orthogonal transmit diversity        OTD, orthogonal space-time transmit diversity STTD processing, a        non-orthogonal space-time transmit diversity STTD processing,        delay diversity DD processing, Space-Time Trellis-Code        processing, or Space-Time Turbo-Code processing,    -   said input symbol sequence is a channel coded sequence,    -   said channel coding is Turbo coding, convolutional coding, block        coding, or Trellis coding,    -   said pre-diversification step/means subjects said input symbol        to more than one of said processings, said processings being        performed in concatenation,    -   said phase offsets in parallel channels differ by a fixed        amount,    -   said phase offsets in parallel channels differ by a maximum        possible amount,    -   said phase offsets in parallel channels cover a full complex        circle of 360°,    -   said phase offsets in parallel channels are taken from a Phase        Shift Keying configuration,    -   said used phase offsets are signaled to the receiver,    -   said phase offsets are at least partially controlled by the        receiver via a feedback channel,    -   all columns of the symbol matrix contain the same symbols,    -   said symbol matrix is an orthogonal space-time block code,    -   said symbol matrix is a non-orthogonal space-time block code,    -   at least one column of the symbol matrix is different from        another column,    -   said symbol matrix contains at least two space-time code        matrices, each modulating different symbols,    -   all columns of the symbol matrix have different symbols, each        parallel channel transmits from respective spatial channel in        parallel at least two symbols allocated to the spatial channel.

Still further, for example, said spreading codes are scrambeled with atransmission unit specific scrambling sequence (e.g. same for allantennas in one base station or transmission unit), said weighting meansapplies a complex weighting matrix [having in its diagonal thetime-invariant phase shift amounts] for the respective symbol sequences,said symbol sequences modulating the respective parallel channels.

Also, for example, said pre-diversification means/step subjects saidinput symbol to at least one of an orthogonal transmit diversity OTDprocessing, parallel transmission (in this case pre-diversificationtakes as an input e.g. 4 different symbols, performs serial to parallelconversion and transmits the 2 symbols in parallel simultaneously fromtwo antennas or branches, this increasing the data rate by factor oftwo), orthogonal space-time transmit diversity (STTD, assuming arbitrarynumber of outputs), a non-orthogonal space-time transmit diversity STTDprocessing (maintaining the rate at 1 or increase the rate beyond 1, butallowing some self-interference), delay diversity DD processing,Trellis-Code processing, Convolutional Code Processing or Turbo-Codeprocessing, said pre-diversification means subjects said input symbol tomore than one of said processings, said processings being performed inconcatenation. So, there exists a system in which there is first channelcoding, e.g. by a Turbo/Convolutional code, the output is given to OTD,S/P, STTD or NO-STTD, and there is typically interleaving after Turbocoding.

Thus, by virtue of the present invention the above mentioned drawbacksinherent to known prior art arrangements are removed.

In particular, the following advantages can be achieved:

-   -   improvement of performance with burst transmission,    -   no interference with channel estimation,    -   simple to implement at the transmitter, as no semi-continuous        sweep is required,    -   no interleaving over multiple frequencies is required for full        benefit,    -   no guard intervals are required in order to be able to correctly        combine the transmitted (distributed) symbol parts at the        receiving side.

In particular, one embodiment of the present invention achieves toobtain similar effective received channels (eq. 1, defined over time)even without making use of phase-hopping, and even if the transmissioninterval is very short and highly parallel burst transmission is used.In another embodiment the invention achieves to randomize thecorrelations within a non-orthogonal space-time code even if thetransmission interval is very short.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail with reference to the accompanying drawings, in which

FIG. 1 shows a simplified block diagram of a basic configuration of adiversity transmitter according to the present invention and operatingaccording to the basic diversity transmission method according to thepresent invention, and

FIG. 2 shows a modification a simplified block diagram of a modifiedconfiguration of a diversity transmitter according to the presentinvention, including a pre-diversification means.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the diversity transmitter and method according to the presentinvention there are multiple parallel transmissions out of at least twospatial channels (which may be antennas or beams). Namely, basicallythere need to be at least two (logical) parallel channels (e.g.spreading codes) allocated to a given user (receiver), the parallelchannels are transmitted out of at least two spatial channels such astransmit antennas (or beams), e.g. like in Trombi, at least two(preferably 8, according to Trombi results) of the parallel channels,transmitted out of at least one of said spatial channel (antenna) areweighted by multiplication with a fixed complex weight, the complexweight being different for at least two parallel channels.

This will be explained in greater detail with reference to FIG. 1.

FIG. 1 shows a diversity transmitter, i.e. a simplified block diagram ofa basic configuration of a diversity transmitter according to thepresent invention. Transmissions out of antennas (representing anexample of spatial channels) A1, . . . , Am are experiencing theinfluence of respective transmission channels a1, . . . , am beforereception at a receiver. In general, a symbol b to be transmitted isprocessed at the transmitter, transmitted via the transmissionchannel(s) and received at the receiver, where it is subjected to areception processing in order to reconstruct the initially transmittedsymbol b. Reception processing involves channel estimation in order tocompensate for the influence of the transmission channels. (Note thatthe symbol b as well as the transmission channels, i.e. channel impulseresponse a thereof, are in matrix notation).

The diversity transmitter comprises a transmit symbol input means 1 forinputting the symbol b (symbol matrix or a sequence of symbol matrices)to be forwarded to a transmit processing means 2. The transmitprocessing means 2 in turn comprises supplying means 2 a for supplyingsaid symbol b to a plurality of at least two branches, each branch beingsupplied to a respective one of transmit antennas A1, Am (i.e. spatialtransmit channels) for transmission to a receiver. A parallelizationmeans 2 b is adapted to provide within each branch at least two parallelchannels allocated to a respective user (i.e. receiver), and a weightingmeans 2 c is adapted to multiply the symbol signals on at least one ofsaid branches with a fixed complex weight, the complex weight beingdifferent for at least two parallel channels.

In FIG. 1, the parallelization is performed in each of the shown twobranches, while the weighting is effected only in the lower branch.Nevertheless, it may be performed in the upper branch instead of thelower branch or in both branches.

The parallelization means 2 b is adapted to perform a multicodetransmission WCDMA such as for example Hadamard transformation bymultiplying with a user and/or service specific spreading code matrix H,and the spreading code matrix may be antenna specific. In theillustrated example, however, the same spreading code matrix has beenselected for each antenna or spatial channel. If a Hadamardtransformation is applied for spreading, scrambling prior to outputtingthe processed symbols to the antennas is performed, while however, suchscrambling and RF processing is omitted from the illustration forpurposes of keeping the illustration simple.

The received signal r at the receiver after multipath propagation overthe channels a1, a2 (am with m=2) is thenr=a ₁ Hb+a ₂ Hdiag([exp(jφ ₁), . . . ,exp(jφ _(n))])b+n  (2)where the diagonal matrix “diag” consists of the fixed complex weightsof a weighting matrix W, and coefficients a1, a2 refer to channelcoefficients (i.e. the respective channel impulse response) between agiven antenna and the terminal H is the matrix of spreading codes, nrepresents a noise component, and b the vector/matrix of transmittedsymbols

This basic embodiment may be concatenated with pre-diversificationperformed by a pre-diversification means 3 arranged downstream saidinput means 1 and upstream said transmit processing means 2.

FIG. 2 shows an example of such a diversity transmitter, with the inputmeans being omitted from the figure. The pre-diversification means 3,for example performing STTD or another diversity concept, is adapted tosubject said inputted symbol matrix b to a diversification, eachdiversified symbol (vectors/sequences) b1, b2 being supplied to saidprocessing means. As shown in the example in FIG. 2, a first diversifiedsignal b1 is supplied to a processing means 21, while a seconddiversified signal b2 is supplied to a processing means 22. Means 21 and22 are illustrated as being identical to each other, while this is notrequired to be. They may differ in terms of the spreading matrix H used,the weighting matrix W used, and even in the number m of spatialchannels (antennas or beams) used. In a multiuser CDMA system where atleast one transmission applies said multicode transmission it ispreferred that the means 21 and 22 are the same, but that differentusers select different codes from said spreading matrix. Then thedifferent code-multiplexed users do not interfere with each other, ifthe matrix is orthogonal.

When concatenating this with other transmission diversity concepts asshown in FIG. 2, one needs to define the symbols (b) differently. Inparticular, we can consider a case in which we have 4 antennas, and inwhich two parallel transmissions (both transmitting from two antennas)are Combined with Orthogonal Transmit Diversity (OTD), such that theyhave the same symbol sequences, but different antenna specificorthogonal spreading matrices H1 and H2. Then one receives a signalr=a ₁ H ₁ b+a ₂ H ₁diag([exp(jφ ₁), . . . ,exp(jφ _(n))])b+a ₃ H ₂ b+a ₄H ₂diag([exp(jφ ₁), . . . ,exp(jφ _(n))])b+n  (3)with four subscripts 1, . . . 4 for the channels as in the example thereare m=4 transmission antennas, two subscripts 1, 2, for parallel(multi)codes H1, H2 in different transmission antenna branches (due toOTD). In this case they are preferably different submatrices of aHadamard code matrix, and thereby orthogonal to each other. n againrepresents a noise component.

In alternative concepts we have at least partly different symbolssequence/vectors b1 and b2 transmitted from the two different antennapairs. Then, the received signal is given byr=a ₁ H ₁ b ₁ +a ₂ H ₁diag([exp(jφ ₁), . . . ,exp(jφ _(n))])b ₁ +a ₃ H ₂b ₂ +a ₄ H ₂diag([exp(jφ ₁), . . . ,exp(jφ _(n))])b ₂ +n  (3′)

For example, the symbol vectors b1 and b2 may represent two differentinformation substreams (possibly after channel coding), formed withserial to parallel (S/P) conversion. Then, the transmission conceptincreases the data rate by a factor of two. If the spatial channels fromthe transmit antennas to the receive antennas are sufficiently differentwe can have H1=H2 and still be able to detect the symbol streams b1 andb2. In alternative embodiments, the symbol streams b1 and b2 may eachbelong to different branches of the STTD (or any other space-time code,including Space-time spreading). As an example, with STTD, the b1=(c1c2) and b2=(−c2* c1*), where c1 and c2 are the (complex) symbolsforwarded to parallel transmission means. In this case these symbolsequences are separable (in fact, orthogonal) due the properties of thespace-time code, and the matrices H1 and H2 are preferably identical.Note that in this case the symbol rate remains at 1 since it takes twotime intervals to transmit two symbols. However, the diversity order isdoubled. The code matrices are preferably orthogonal, e.g. Hadamardcodes, or rotated (scrambled) Hadamard codes, or nearly orthogonal, suchas well known Gold codes. Any other orthogonal space-time code can beused.

It is to be noted that the antenna specific parallel channels H, H1 andH2 above can have an arbitrary spreading factor, and can be implementedin code domain (parallel codes), or with orthogonal carriers(frequencies).

This can be combined with any other transmit diversity concept, when e.gextending the number of transmit antennas further. One can concatenatedelay diversity, non-orthogonal space-time codes, orthogonal space-timecodes, space-time trellis codes, Turbo-coded transmit diversity andvarious others with the proposed concept. Also, if one does not have asufficient number of parallel channels but a sufficient number ofsuccessive symbol intervals, one can still use phase hopping, as theprior art proposes.

The complex weights should be defined so that consecutive coded bits orsymbols see a different channel. In one example, the parallel channelsapply phases with 0, 180, 90, −90 degree offsets. Preferably, we wouldhave at least eight states (e.g. from PSK alphabet), such that the phasesequence visits all states once and such that the “path length” ismaximized.

One can apply the fixed phase coefficients also in analogy with the ABBAconcept and/or randomized ABBA concept (RABBA) if there are at leastthree transmit paths, e.g. three different transmit antennas. This isthen effected in the pre-diversification means 3. In this case asequence of symbols is input to the ABBA code (e.g. as in Trombi), eachoutput of the ABBA code (columns of the ABBA, or some othernon-orthogonal space-time code matrix) are directed to differenttransmit antennas, and the sequence in each different branch issubjected to multicode transmission, such that at least in one branch atleast one code is subjected to a fixed phase rotation. Note, that thiscan be implemented also so that selected symbols in a given branch ofthe non-orthogonal code have a rotated signal constellation.

It is assumed that there are four antennas, and that different columnsof the ABBA code are transmitted out of antennas 1, . . . , 4 inparallel with different K spreading (Walsh) codes (w_k) or carriers(e.g. vectors from the IFFT matrix in OFDM). In this case thetransmitted signal for a packet of 4*K (number 4 comes from theparticular ABBA construction, where in this example the 2×2 matrices Aand B both contain 2 symbols e.g. from QPSK alphabet, there isessentially the same symbol construction in STTD, described earlier)symbols are given by equation (4) below $\begin{matrix}{\sum\limits_{k = {1:K}}\begin{bmatrix}{w_{k}A_{k}} & {w_{k}{\exp\left( {j\theta}_{k} \right)}B_{k}} \\{w_{k}B_{k}} & {w_{k}{\exp\left( {j\theta}_{k} \right)}A_{k}}\end{bmatrix}} & (4)\end{matrix}$where matrices A and B are the elements of the so called ABBA matrix(ABBA can be replaced here by any other non-orthogonal block code,optimized for one of multiple receive antennas). Each paralleltransmission, indexed by k, carries different ABBA symbol matrices, andthe signals are transmitted out of antennas.

In the aforementioned example several parallel ABBA transmissions havedifferent complex weights modulating the output of at least one antenna(in the example in previous equation, two antennas).

Thus, there is no time index in the complex phasors (phasors means themultiplication factor achieving the time-invariant phase shift). Notethat the parallel Trombi-concept can be described also with eq. 4, whentwo (and only two) B and A matrices switch place (are exchanged).

In terms of other conceivable embodiments it is clear that a similarapproach can be used for any system with more than two antennas. Also,the number of antennas need not to be an even number as in theillustrated examples in FIG. 1 and 2, but may be arbitrary.

The parallel weighted channels can be transmitted to non-fixed beamswhich are defined for example by long-term feedback from the receiver orby receive measurements, or by both; or they can be transmitted to fixedbeams.

The complex weights do not need to have unit norm but may have a valuediffering from 1.

The previous phase-hopping concept has been converted according to thepresent invention to a phase-modulation concept which is applicable e.g.to the parallel channels in HDR (High Data Rate) or to any high ratetransmission concept in which a number of parallel channels are usedwith at least two transmit antennas.

This proposed concept has the advantage that it avoids the previoustime-variant phase-offset arrangements, thereby resulting in a simplerreceiver implementation in the user equipment. The concept can beimplemented at base band using different rotated symbol constellationsin parallel channels prior signal spreading.

The performance of the concept, when combined with STTD is similar aswith Trombi, when this is used in place of phase hopping.

Best symbol error rate is likely to be obtained with ABBA based/typesolution with fixed phase offsets (in the presence of at least 3antennas/channels). In that case one may be able to use a very high ratechannel code, and some ARQ solution for “good enough” performance (withARQ the phase can change if the retransmission occurs well with thechannel coherence time).

With OFDM, the ABBA solution randomizes the interference across multipleantennas, and typically requires that a simple (linear or nonlinear)interference cancellation concept is used in the receiver to mitigate tonon-orthogonality of the space-time code. With parallel-Trombi, it issimpler, as the codes and symbols remain orthogonal. No such CDMA basedsystem (HDR, HSDPA) without the space-time code component even for twotransmit antennas achieving the advantages described herein before ispresently known to the inventors. In any case it is beneficial then touse one of the parallel channels for channel estimation and fix thephase offset for the parallel channels a priori so that the receiver ofthe user equipment need not estimate the channel for each of theparallel channels separately. However, if the parallel channels haveindependent channel estimators, the proposed concept is backwardcompatible to such a system. It is also possible that the transmittercan use the concept if it so desires and the receiver blindly detectsthis. Blind detection can be done for example by demodulating the signalusing the proposed concept (fixed phase offset) and without using theproposed concept, and selecting the one that gives better performance(e.g. symbol reliabilities at the output of the decoder).

It is likely that the performance increase will be the highest in indoorchannels (7-9 dB's compared to single antenna transmission).

In particular, it is to be noted that the parallelization means isadapted to achieve a multicode transmission (WCDMA, Wideband CodeDivisional Multiple Access). In this case, the parallelization meansuses a Hadamard transformation (Hadamard matrix) or a submatrix of aHadamard matrix. Also, parallelization is then followed by scramblingprocessing prior to transmission out of the antennas. If no Hadamardmatrix for prallelization is to be used, so-called Gold codes may beused instead, which alleviates tie necessity for subsequent scrambling.With scrambling, however, better autocorrelation properties for thesignal can be obtained to make use of RAKE diversity.

Alternatively, the parallelization means may be adapted to perform anInverse Fast Fourier Transformation IFFT (in connection with OFDM).

Furthermore, it is pointed out that the weighting means are adapted toperform not only a simple multiplication but a linear transformation. Tothis end, the weighting matrix may preferably be a matrix of the kindsuch that the weighting matrix W is unitary, i.e. that when multipliedwith its conjugate complex transposed matrix W^(H) yields the identitymatrix I having only values of “1” in its diagonal (W^(H)*W=I).

Moreover, in case the pre-diversification means subject the symbol to anon-orthogonal space-time code, non-orthogonal space-time block codessuch as ABBA, Randomized ABBA (RABBA), or Alamouti Code (STTD in WCDMA)are advantageously to be used.

Also, the sequence of parallelization and weighting may be reversed,provided that the used signal processing is suitably modified. Thesymbols prior parallel transmission can be subjected to weighting or theparallel channels (modulated spreading codes) can the subjected toweighting.

Thus, according to the present invention, the phases in (selectedparallel channels) in m−1 out of m antennas are randomized by fixedcomplex weights so that destructive combination does not dominate. If nparallel channels are present due to parallelization, a n dimensionalweighting matrix can be used, although a two-dimensional matrix could besufficient. In an n dimensional matrix, some of the complex phases maybe set to zero, in which case no weighting is imposed on that particularchannel.

It is to be noted that signals as described in this application arerepresented in matrix notation. Thus, the symbol and/or symbol sequenceis to be understood to be in matrix notation. Those matrices are forexample described in “Complex Space-Time Block Codes For Four TxAntennas”, O. Tirkkonen, A. Hottinen, Globecom 2000, December 2000, SanFrancisco, US for orthogonal space-time codes with a different number oftransmit antennas. (A single symbol would correspond to a sequence of aminimum sequence length, e.g. length one).

Also, a receiver adapted to receive the signals transmitted according tothe present invention will have to be provided in particular with acorresponding despreading functionality, adapted to perform the inverseoperation as compared to the transmitting side. This inverse operationcan be expressed by the subjecting the received signal to an inverseprocessing, represented by multiplying with the inverse matrices (H⁻¹,W⁻¹). Apart therefrom, a receiver will comprise RF parts, a channelestimator, a symbol detector (e.g. based on maximum likelihood, maximuma posteriori, iterative interference cancellation principles), andmultiplexer.

Accordingly, as has been described herein above, the present inventionconcerns a diversity transmitter, comprising: transmit symbol inputmeans (1) for inputting a symbol matrix (b) to be forwarded to atransmit processing means (2), said transmit processing means comprisingsupplying means (2 a) for supplying said columns of said symbol matrixto a plurality of at least two branches, each branch being supplied to arespective one of spatial channels (A1, . . . , Am) for transmissin to areceiver, a parallelization means (2 b) adapted to provide within eachbranch at least two parallel channels allocated to a respective user,and weighting means (2 c) adapted to subject the symbol matrix signalson at least one of said branches to an invertible linear transformationwith a fixed complex weight, the complex weight being different for atleast two parallel channels. The present invention also concerns acorresponding diversity transmission method.

Although the present invention has been described herein above withreference to its preferred embodiments, it should be understood thatnumerous modifications may be made thereto without departing from thespirit and scope of the invention. For example, the transmitter may bethe mobile terminal, and the receiver the base station, or anothermobile terminal. Furthermore, the spatial channel may include so calledpolarization diversity channels. Power control may be applied to theparallel channels separately or jointly. Some of the parallel channelsmay provide a different quality of service (e.g. BER) and have differentchannel coding (error correction/error detection), and/or differentrelative transmit powers. Some of the transmit antennas may belong todifferent base stations in which case the invention can be used toprovide macrodiversity or soft handoff. It is intended that all suchmodifications fall within the scope of the appended claims.

Documents cited in the Specification:

-   [1] “A Randomization Technique for Non-Orthogonal Space-Time Block    Codes”, by A. Hottinen & O. Tirkkonen, presented on IEEE Vehicular    Technology Conference, May 2001, Rhodes, Greece-   [2] “New CPICH Transmission Scheme for 4-antenna transmit    diversity”, Samsung contribution to TSG-RAN Working Group 1 meeting    #15, Berlin, Germany, Aug. 22-Aug. 25, 2000-   [3] “A Space-Time Coding Concept for a Multi-Element Transmitter”,    by A. Hottinen, K. Kuchi, O. Tirkkonen, Canadian Workshop on    Information Theory, June 2001, Vancouver, Canada-   [4] “Spatial Transmit diversity techniques for broadband OFDM    systems”, by S. Kaiser, published in IEEE, 2000, page 1824-1828,    (0-7803-6451-1/00)-   [5] U.S. Pat. No. 6,157,612-   [6] “Draft Baseline Text for Physical Layer Portion of the 1xEV    Specification”, 3GPP2 C.P9091 ver. 0.21, Aug. 24, 2000 (3GPP2 TSG-C    working group III)-   [7] “Complex Space-Time Block Codes For Four Tx Antennas”, O.    Tirkkonen, A. Hottinen, Globecom 2000, December 2000, San Francisco,    Us

1. A diversity transmitter, comprising: transmit symbol input means forinputting a symbol matrix to be forwarded to a transmit processingmeans; the transmit processing means comprising supplying means forsupplying columns of the symbol matrix to a plurality of at least twobranches, each branch being supplied to a respective one of spatialchannels for transmission to a receiver; a parallelization means forproviding within each branch at least two parallel channels allocated toa respective user; and weighting means for subjecting the symbol matrixsignals on at least one of the branches to an invertible lineartransformation with at least one fixed complex weight, the complexweight being different for at least two parallel channels.
 2. Adiversity transmitter according to claim 1, wherein: the invertiblelinear transformation is a unitary transformation.
 3. A diversitytransmitter according to claim 2, wherein: the unitary transformation isrepresented by a unitary weight matrix in which at least two elementshave different non-zero complex phase values.
 4. A diversity transmitteraccording to claim 1, wherein: the parallelization means performsmulticode transmission using multiple spreading codes.
 5. A diversitytransmitter according to claim 4, wherein: multicode transmission isperformed using a Hadamard transformation by multiplying the symbolswith a spreading code matrix.
 6. A diversity transmitter according toclaim 4, wherein: the spreading code matrix is antenna specific.
 7. Adiversity transmitter according to claim 4, wherein: the spreading codesare non-orthogonal spreading codes.
 8. A diversity transmitter accordingto claim 4, wherein: the spreading codes are orthogonal spreading codes.9. A diversity transmitter according to claim 1, wherein: the fixedcomplex weights applied by the weighting means are time-invariant phaseshift amounts for the respective parallel channels.
 10. A diversitytransmitter according to claim 9, wherein: the phase shift amounts areindependent of the channels in at least two corresponding parallelchannels transmitted out of different antennas.
 11. A diversitytransmitter according to claim 9, wherein: the phase shift amounts aredependent on the channels.
 12. A diversity transmitter according toclaim 9, wherein: the weighting matrix is identical for each branch. 13.A diversity transmitter according to claim 9, wherein: the weightingmatrix differs for each branch.
 14. A diversity transmitter according toclaim 1, comprising: a pre-diversification means arranged downstream theinput means and upstream the transmit processing means; thepre-diversification means subjects the inputted symbol sequence to adiversification, at least one diversified symbol sequence being suppliedto the processing means.
 15. A diversity transmitter according to claim14, wherein: the pre-diversification means subjects the input symbolsequence to at least one of an orthogonal transmit diversity, orthogonalspace-time transmit diversity processing, a non-orthogonal space-timetransmit diversity processing, delay diversity processing, Space-TimeTrellis-Code processing, or Space-Time TurboCode processing.
 16. Adiversity transmitter according to claim 1, wherein: the input symbolsequence is a channel coded sequence.
 17. A diversity transmitteraccording to claim 16, wherein: the channel coding is Turbo coding,convolutional coding, block coding, or Trellis coding.
 18. A diversitytransmitter according to claim 15, wherein: the pre-diversificationmeans subjects the input symbol to more than one of the processings, theprocessings being performed in concatenation.
 19. A diversitytransmitter according to claim 9, wherein: the phase offsets in parallelchannels differ by a fixed amount.
 20. A diversity transmitter accordingto claim 9, wherein: the phase offsets in parallel channels differ by amaximum possible amount.
 21. A diversity transmitter according to claim9, wherein: the phase offsets in parallel channels cover a full complexcircle of 360°.
 22. A diversity transmitter according to claim 9,wherein: the phase offsets in parallel channels are taken from a PhaseShift Keying configuration.
 23. A diversity transmitter according toclaim 9, wherein: the used phase offsets are signaled to the receiver.24. A diversity transmitter according to claim 9, wherein: the phaseoffsets are at least partially controlled by the receiver via a feedbackchannel.
 25. A diversity transmission method, comprising the steps of:inputting a symbol matrix for being processed, the processing comprisingsupplying columns of the symbol matrix to a plurality of at least twobranches, each branch being supplied to a respective one of spatialchannels for transmission; performing parallelization so as to providewithin each branch at least two parallel channels allocated to arespective user; and subjecting the symbol matrix signals on at leastone of the branches to an invertible linear transformation with at leastone fixed complex weight, the complex weight being different for atleast two parallel channels.
 26. A method according to claim 25,wherein: the invertible linear transformation is a unitarytransformation.
 27. A method according to claim 26, wherein: the unitarytransformation is represented by a unitary weight matrix in which atleast two elements have different non-zero complex phase values.
 28. Amethod according to claim 25, wherein: the parallelization performsmulticode transmission using multiple spreading codes.
 29. A methodaccording to claim 28, wherein: multicode transmission is performedusing a Hadamard transformation by multiplying the symbols with aspreading code matrix.
 30. A method according to claim 28, wherein: thespreading code matrix is antenna specific.
 31. A method according toclaim 28, wherein: the spreading codes are non-orthogonal spreadingcodes.
 32. A method according to claim 28, wherein: the spreading codesare orthogonal spreading codes.
 33. A method according to claim 25,wherein: the fixed complex weights applied by the weighting means aretime-invariant phase shift amounts for the respective parallel channels.34. A method according to claim 33, wherein: the phase shift amounts areindependent of the channels in at least two corresponding parallelchannels transmitted out of different antennas.
 35. A method accordingto claim 33, wherein: the phase shift amounts are dependent on thechannels.
 36. A method according to claim 33, wherein: the weightingmatrix is identical for each branch.
 37. A method according to claim 33,wherein: the weighting matrix differs for each branch.
 38. A methodaccording to claim 25, comprising: a pre-diversification step performedafter inputting and before processing; the pre-diversification stepsubjects the inputted symbol sequence to a diversification, at least onediversified symbol sequence being subjected to the processing.
 39. Amethod according to claim 28, wherein: the pre-diversification stepsubjects the input symbol sequence to at least one of an orthogonaltransmit diversity, orthogonal space-time transmit diversity processing,a non-orthogonal space-time transmit diversity processing, delaydiversity processing, Space-Time Trellis-Code processing, or Space-TimeTurboCode processing.
 40. A method according to claim 25, wherein: theinput symbol sequence is a channel coded sequence.
 41. A methodaccording to claim 40, wherein: the channel coding is Turbo coding,convolutional coding, block coding, or Trellis coding.
 42. A methodaccording to claim 39, wherein: the pre-diversification step subjectsthe input symbol to more than one of the processings, the processingsbeing performed in concatenation.
 43. A method according to claim 33,wherein: the phase offsets in parallel channels differ by a fixedamount.
 44. A method according to claim 33, wherein: the phase offsetsin parallel channels differ by a maximum possible amount.
 45. A methodaccording to claim 33, wherein: the phase offsets in parallel channelscover a full complex circle of 360°.
 46. A method according to claim 33,wherein: the phase offsets in parallel channels are taken from a PhaseShift Keying configuration.
 47. A method according to claim 33, wherein:the used phase offsets are signaled to the receiver.
 48. A methodaccording to claim 33, wherein: the phase offsets are at least partiallycontrolled by the receiver via a feedback channel.
 49. A diversitytransmitter according to claim 1, wherein: all columns of the symbolmatrix contain identical symbols.
 50. A diversity transmitter accordingto claim 1, wherein: the symbol matrix is an orthogonal space-time blockcode.
 51. A diversity transmitter according to claim 1, wherein: thesymbol matrix is a non-orthogonal space-time block code.
 52. A diversitytransmitter according to claim 1, wherein: at least one column of thesymbol matrix is different from another column.
 53. A diversitytransmitter according to claim 1, wherein: the symbol matrix contains atleast two space-time code matrices, each modulating different symbols.54. A diversity transmitter according to claim 1, wherein: all columnsof the symbol matrix have different symbols, each parallel channeltransmits from respective spatial channel in parallel at least twosymbols allocated to the spatial channel.
 55. A method according toclaim 25, wherein: all columns of the symbol matrix contain identicalsymbols.
 56. A method according to claim 25, wherein: the symbol matrixis an orthogonal space-time block code.
 57. A method according to claim25, wherein: the symbol matrix is a non-orthogonal space-time blockcode.
 58. A method according to claim 25, wherein: at least one columnof the symbol matrix is different from another column.
 59. A methodaccording to claim 25, wherein: the symbol matrix contains at least twospace-time code matrices, each modulating different symbols.
 60. Amethod according to claim 25, wherein: all columns of the symbol matrixhave different symbols, each parallel channel transmits from respectivespatial channel in parallel at least two symbols allocated to thespatial channel.